I'll add one idea to the comments already made. The environment
where intersymbol interference occurs is typically linear (lossy
transmission lines, sometimes with reflections). Hence, a single
step response theoretically tells the whole story. From this,
an eye diagram can be constructed by time-shifting the step response
by multiples of the bit time (1/data rate) and adding. (You have to
use both the positive and negative step response, of course.)

This is more practical than it might sound. It takes only a few
time-shifted waveforms to get a pretty clear picture, typically.
And, along the way, what "intersymbol interference" means is
clarified. For example, reflected noise from previous data edges
getting time shifted into the data eye.